(1) Field of the Invention
The present invention relates to an optical device, in particular relating to an optical waveguide filter and a production process thereof.
(2) Description of the Prior Art
Conventionally, a fiber grating producing method has been the technology used for producing devices for wavelength filters in a fiber.
In general, gratings have two types, namely, radiation gratings and reflection gratings. As shown in FIG. 1, the former couples the mode (.beta.) that propagates through the core, with the cladding mode (.beta.clad); the latter couples the mode (.beta.) that propagates in the positive direction through the core, with the mode (.beta.ref) that propagates in the negative direction. This coupling is enabled by a perturbation arising in the core.
In a typical optical fiber, this perturbation is caused by variation in refractive index. The variation in refractive index occurs periodically at intervals of some 100 .mu.m for a radiation type, and at intervals of about 1 .mu.m for a reflection type so as to allow coupling between the modes.
For a grating in a planar waveguide, variation in refractive index of the core was formed after the production of the waveguide. As seen in Japanese Patent Application Laid-Open Hei 7 No. 281,016, for example, for a reflection type waveguide, variation in refractive index of the core is formed after the production of the waveguide. In this publication, as shown in FIG. 2A, a waveguide is formed of a core a and a cladding b. This is modified by the addition of some impurity such as germanium (Ge) oxide etc. The resultant is illuminated across a prescribed range c with ultraviolet light with interference fringes, so as to produce a diffraction grating in which the refractive indices of core a and cladding b within the prescribed range c are changed periodically in the beam propagating direction, as shown in FIG. 2B.
Up to now, however, no radiation grating has been reported to be successfully produced using a planar waveguide because of difficulties in its fabrication etc., as will be described next. In the conventional art, silica substrates were usually used. When the refractive index was modified after the production of the waveguide, the substrate was left for some days in a hydrogen gas atmosphere under a pressure of 100 atm so as to diffuse hydrogen into the silica, thus causing variation in refractive index of the core. That is, this process needed several days after the production of waveguide chips.
Since the presence of hydrogen increases the refractive indices of the cladding and core, the properties of the grating change some days after its production. This change should be taken into account when it is fabricated.
For a radiation grating, the grating pitch is of some 100 .mu.m, but it is difficult to control the wavelength center (within 1 nm) unless the accuracy of forming the grating is of sub-micron level.
The mask for ultraviolet light to be used for a photolithographic process, will be damaged by ultraviolet irradiation. This limits the material for the mask. That is, chromous masks, which are used for semiconductor processes etc., can not be used. Further, the thickness of the mask should be made large considering the damage by the irradiation. From this requirement, the accuracy in the fabrication of the masks of stainless steel which are now commonly used is of .+-.5 .mu.m.
Even if the accuracy of the mask was within 1 .mu.m, it is still difficult to control the wavelength center if the energy density of ultraviolet light is poor in its uniformity. Further, other than the wavelength center, if there is degradation of constituent parameters for each part of the grating, the inhibiting band of the grating becomes wider, making it impossible to obtain the designated band width.
When the reflecting and absorbing performances of the substrate are considered, silica is most preferable for the material of the substrate. For example, when a Si-substrate was used, abrasion damage occurred due to the processing by laser with an energy density of 1 mJ/mm.sup.2. In order to prevent this, it is necessary to reduce the energy density to 1/5 of the present level.
For the case of a planar waveguide, since the area of it is greater than that of the fiber, the number of chips which can be produced per each laser irradiation is limited to five or below (for a fiber, 30 or more can be produced).